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Scientists use alfalfa plants to harvest nanoparticles of gold

15.08.2002

Ordinary alfalfa plants are being used as miniature gold factories that one day could provide the nanotechnology industry with a continuous harvest of gold nanoparticles.

An international research team from the University of Texas-El Paso (UTEP) and Mexico advanced the work at the Stanford Synchrotron Radiation Laboratory (SSRL) - part of the Stanford Linear Accelerator Center (SLAC) in Menlo Park, Calif. The researchers are using, as tiny factories, the alfalfas natural, physiological need to extract metals from the medium in which they are growing. Of most value here is that the alfalfa extracts gold from the medium and stores it in the form of nanoparticles - specks of gold less than a billionth of a meter across. Their findings are published in the April issue of Nano Letters, a publication of the American Chemical Society.

This study is just one of hundreds of innovative research projects that take advantage of the unique properties of synchrotron X-rays provided by SSRL, said Keith Hodgson, the director of SSRL and a professor of chemistry at Stanford University.

The semiconductor industry has long valued the oxidation resistance and the thermal and electrical conductivity of gold. Now, the relentless and accelerating drive toward ever-smaller wires, connectors and through-holes on ever-smaller semiconductor devices makes those properties even more important to the folks who make very small things. Consequently, the nanotechnology industry is very interested in processes that make gold nanoparticles for nano-scale electronic and optical devices.

Gold factories

Some processes for making high concentrations of solid nanoparticles use chemistry. However, many of the chemical methods used are cumbersome and lead to poisonous end products that could endanger public health. A better method is needed. Ordinary plants, acting as tiny factories, may provide a better means of production.

All ordinary plants use their roots to extract nutrients - water and minerals, even heavy metals - from the soil they grow in. In this case, alfalfa was chosen as a model plant system for studying the ability of plants to extract gold from various growth media. If it worked, using plants to produce gold nanoparticles would eliminate the need for harsh chemicals or chemical reducing agents. As this would be a significant environmental advantage, the Environmental Protection Agency, the Department of Energy (DOE) and the National Institutes of Health provided funding.

The researchers needed answers to such questions as: Will plants make gold nanoparticles? And if they will, how then do we determine the presence, size and physical distribution of nanoparticles within the plant? And how will we extract the nanoparticles from the alfalfa?

A University of Texas (UT) team from the El Paso and Austin campuses is working on those answers in Texas and at SSRL. Work to date demonstrates answers to the first two of those questions, and the team is working on answering the third.

Subatomic scale

The alfalfa plants were germinated and grown on an artificial, gold-rich, soil at UTEP. At special SSRL facilities, a UT team led by chemistry and environmental sciences Professor Jorge Gardea-Torresdey analyzed the samples using X-ray absorption spectroscopy (XAS), which allows for selective study of metals and their chemical environments in biological samples on a subatomic scale. Because metals play an important role in biological systems - many are beneficial or even essential - XAS is a powerful tool for accurate characterization of the metal environment in metalloproteins and for a better understanding of their function.

The researchers also analyzed the samples using high-resolution transmission electron microscopy. The advantage electron microscopes offer over light microscopes is about a thousand-fold increase in resolution and a hundred-fold increase in the depth of field. All the electron microscope work was done at UT-Austin.

The UT-Austin microscopy team, directed by chemical engineering Professor Miguel Jose Yacaman, formerly of the Institute of Physics at UNAM in Mexico, produced the first answer when electron microscopy images of the alfalfa shoots confirmed the existence of gold nanoparticles in the roots and along the entire shoot of the alfalfa plants. The second answer appeared when SSRL imaging equipment corroborated the presence of gold and determined that the particles have physical properties that are similar to gold nanoparticles formed using chemical techniques. The answer to the third question - how to extract the nanoparticles from the plants - may be by centrifuge, which may turn out to be easier than expected.

International collaboration

This collaboration between two important universities in the UT system - the team conducting microscopy work at UT-Austin collaborates with the phytoremediation group at UTEP to investigate nanotechnology issues through the XAS capability at SSRL - is great, Gardea-Torresdey and Yacaman said. Phytoremediation uses plants for the biological remediation of environmental problems.

Two international postdoctoral fellows participated in the Yacaman group: Patricia Santiago from the Instituto Nacional De Investigaciones Nucleares de Mexico (ININ), and Horacio Troiani from the Balseiro Institute in Bariloche, Argentina.

The work has demonstrated that using alfalfa is a cost effective and environmentally friendly method of producing gold nanoparticles. Future work will involve the full physical characterization of the nanoparticles and the development of methods to extract them from the plants, Gardea-Torresdey added.

Last year, more than 1,700 scientists from better than 200 universities and companies used SSRL resources for 775 individual experiments that were carried out at our national user facility, which is funded by the DOE Office of Basic Energy Sciences, Hodgson noted. What they learned advanced frontiers of knowledge in fields from computer chip manufacturing to advancing drug discovery.

The project is funded through the DOE/SSRL Gateway program and the beam time is funded by SSRL. The synchrotron aspects of this project are supported by the DOE/SSRL Gateway program, which is a joint cooperative research and training effort by SSRL and UT-El Paso to engage Mexican- American students and students belonging to ethnic minorities in frontier scientific research using advanced facilities.

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